Electronic systems that operate with RF signals may include an RF transceiver that processes a wireless or wired signal, for example, an RF signal. An RF transceiver may include various components to amplify and/or filter an RF signal to recover original data carried by the RF signal. An increased demand for low-cost and low-power architecture of wireless communication design has focused attention on direct conversion RF transceivers because they simply down-convert by removing intermediate frequency (IF) analog components. However, direct-conversion RF transceivers suffer from an imbalance between analog I and Q branches that arise from the imperfect analog front end (FE) components.
Imperfect IQ down-conversion may generate gain and phase imbalance between I and Q components. Gain mismatch may arise from unequal gains of a mixer, unequal gains for local oscillator (LO) drivers that supply an LO clock to the I and Q branches, unequal gains in variable gain amplifier (VGA) components of I and Q branches, and/or unequal least significant bit (LSB) levels of analog-to-digital converters (ADCs) in the I and Q branches. The phase imbalance primarily arises from a difficulty in achieving a precise 90 degree phase between I and Q clocks. Since these types of imbalance do not depend on signal frequency, they are referred to as a frequency-independent (FI) imbalance.
Analog baseband (ABB) filter pole position mismatch between analog I and Q paths may cause frequency-dependent (FD) IQ mismatch. The FI and FD IQ imbalance results in a mirror image signal in the signal bandwidth. A typical image rejection ratio (IRR) at the receive side ranges from 20 to 40 dB, which is insufficient to correctly receive high-order modulation carriers that require a high signal-to-noise ratio (SNR).
The principal approaches for estimating and compensating for IQ mismatch are a frequency-domain approach and a time-domain approach. The frequency-domain approach significantly reduces the complexity of IQ compensation when compared with the time-domain approach, due to a conversion of convolution operations into multiplication operations in the frequency domain. However, the frequency-domain approach requires special pilot patterns for IQ imbalance estimation, which reduces the spectral efficiency and available throughput due to the overhead of pilot insertion. Thus, the frequency-domain approach is not supported in wireless access network (WAN) standards, such as, for example, Long Term Evolution (LTE) and Wideband Code Division Multiple Access (WCDMA).
The time-division approach uses blind estimation by exploiting an orthogonal property of a received signal to compensate FI and FD mismatch. The time-domain approach does not require special pilot patterns to estimate IQ mismatch, which enables efficient utilization of a wireless channel.